A general X-ray tube generates X-rays by allowing electrons to collide with a metallic anode target with high energy. For example, the X-ray tube uses a generation principle of Bremstralung X-rays or predetermined X-rays generated according to a material of an anode target. Herein, an electron source emitting electrons is generally a thermal electron source.
Meanwhile, an X-ray tube emitting electrons by using nano materials is provided. The X-ray tube uses a field-emission emitter. In the case of the X-ray tube, it is important to apply nano materials, which are effective for field emission, to a cathode electrode, form a gate electrode in order to apply an electric field to the nano materials, and seal each structure of the X-ray tube in a vacuum.
A field emission source has a structure to use electrons emitted from materials by a tunneling effect when the electric field is applied to the emitter, unlike the thermal electron source. A general structure of a field emission source uses a principle in which the electric field is applied to the emitter on the cathode by voltage applied between the cathode and the gate by inserting between the anode and the cathode one or more gate electrodes having a grid or one or more gate holes on the emitter. The plurality of gates are additionally installed between the gate and the anode in addition to a gate inducing field emission to be used to appropriately control a trajectory of an emitted electron beam. When the gate electrode is used as a mesh, it is advantageous in that the emitter and the gate holes do not need to be aligned, but gate current that leaks through the gate holes cannot be prevented.
In order to remove the leakage, the gate holes are aligned according to an emitter pattern and the gate holes need to be maintained at a regular interval. When the field emission source is formed in a large pattern, it is advantageous that the gate holes are aligned in the emitter pattern, but a distance between the gate electrode and the emitter increases. Therefore, it is disadvantageous in that higher voltage is applied to the gate electrode in order to acquire the same field-emission current. That is, when the emitter and the gate holes are formed largely to be aligned by visual inspection, it is easy to manufacture the X-ray tube, but it is disadvantageous in that sufficient field emission occurs only by applying high voltage to the gate.
On the contrary, when the emitter pattern is formed as an array to be smaller and the gate holes are also formed as an array according to the emitter pattern, the cathode and the gate are installed to be closer to each other, thereby reducing gate voltage. That is, in order to reduce voltage applied to the gate, a field-emission emitter is patterned in a small dot array pattern and when the emitter pattern is aligned with gate holes having slightly larger sizes, field emission may occur even at low gate voltage. However, in this case, it is difficult to align the gate holes and the emitter pattern due to the downsized emitter pattern, and as a result, it is difficult to manufacture the X-ray tube. That is, the alignment of the gate holes and the emitter may not be distinguished by visual inspection, and as a result, it is not easy to manufacture the X-ray tube. The gate electrode and the cathode electrode with the emitter need to be insulated from each other while maintaining a predetermined distance. It is not easy to join the gate electrode and the cathode electrode by using a material having small out-gassing which is easily vacuum-sealed with an alignment degree of approximately hundreds of micrometers.
Meanwhile, the X-ray tube using the field-emission emitter should include various electrodes including the gate electrode, the emitter electrode, an anode electrode, and the cathode electrode. The size of the X-ray tube increases due to various electrodes, and as a result, miniaturization is difficult.